In older vane-type vacuum pumps, the rotor and vanes were generally made of steel. However, since such construction resulted in the rotor and vanes being relatively heavy, strong centrifugal forces were created during the operation of the pump which shortened the service life of the device. For this reason, manufacturers have more recently begun making the rotors of sintered metals. Such rotors are advantageous for both production and cost reasons. Furthermore, such rotors are very strong while being both light weight and wear resistant.
However, since oil is used to lubricate the moving parts of the pump, the introduction of sintered metal as the rotor material has resulted a disadvantage that substantially more oil can be absorbed in its pores than with steel. Therefore, at high rotational speeds, substantially more oil will be spun out of the rotor pores and be deposited against the walls of the rotor casing.
Since production techniques have usually required that the inlet opening of the pump extend radially outward through the casing, the separated oil would be thrown into this suction inlet opening. This oil would be deposited on the walls of the inlet opening and creep along them toward the vacuum chamber in spite of the opposed suction effect. Such oil flow has been found to be especially damaging to vacuum systems such as brake energizers since such systems generally have a diaphragm wall which is susceptable to being attacked by the oil, thereby rendering the entire brake energizer system inoperable.